Understand and Measure a Stepper Motor’s Pull-Out and Pull-In Torque

Today’s healthcare, pharmaceutical, and agriculture industries demand faster yet more compact diagnostics equipment that will deliver accurate and repeatable results. Miniature electric motors, along with smaller and inexpensive electronic components, are increasingly used in laboratory and diagnostic equipment designs to help achieve these size and performance goals.

Stepper motors take advantage of brushless technology to offer numerous advantages over DC motors for applications requiring cost-effective and precise positioning. In order to select the right stepper motor for an application, the designer must both understand and account for its pull-out and pull-in torque curves. This blog post will show you how to measure pull-out and pull-in torque while also providing key concepts to consider during stepper motor selection and sizing.

Measure Pull-Out Torque

The pull-out torque curve defines the maximum torque the motor can deliver, when driven at a certain speed, without losing synchronization. Exceeding the pull-out torque will make the motor lose synchronization and produce erratic motion. Typically, the pull-out torque is measured in open loop mode with no load on the motor and with one specific driver, while the motor shaft is connected to a variable brake system. Here’s how:

1.	The motor is started with no load and brought to a given, rather low speed of 100 pulses per second (pps).
2.	An increasing load is applied on the motor shaft using the brake system until the motor loses its synchronization.
3.	The maximum load under which the motor can rotate at 100 pps without losing synchronization is stored.
4.	Steps 1 to 3 are repeated at a higher speed — 200 pps, for example.

The maximum load values for each velocity measured during Step 3 represent the pull-out torque curve of the motor. Due to resonance, certain velocities can cause the motor to behave erratically and should be avoided. A safety factor of approximately 30 percent should be considered for the maximum load torque.

Non-linear acceleration using the maximum available pull-out torque can reach the desired speed faster, thereby optimizing the motor for applications requiring highly dynamic motion. Driving the motor in a closed loop using an encoder allows the engineer to skip the safety factor and use the maximum available pull-out torque of the motor.

Measure Pull-In Torque

The pull-in torque curve defines the maximum speed and torque under which the motor can start without an acceleration ramp. Here’s how to measure pull-in torque when a disk is mounted on the motor shaft, and a cord is wrapped around it:

1.	A low load is applied on the motor.
2.	The driver is set to a low velocity and enabled. If the motor can start, the same step is repeated at a higher velocity.
3.	Once the motor is unable to start, the maximum start-up frequency for the given load has been found.
4.	Steps 1-3 are then repeated with a higher load.

Typically, motor suppliers such as Portescap will provide the pull-in torque curve of the motor without any load torque or inertia and measured with a specific driver. For applications with additional load inertia acting on the motor, we recommend contacting your motor supplier for help calculating the available pull-in torque. In certain cases, the natural resonance frequency of a stepper can be avoided by starting the motor at a speed above the resonance frequency, for which the pull-in torque needs to be known.

Ensure Stepper Motor Performance and Reliability

Understanding the pull-out and pull-in torque curve of a stepper motor is critical to ensuring good performance and reliability in its intended application. Portescap offers a broad range of standard and custom stepper motors along with engineering support. We will help you select a stepper motor with the speed and torque characteristics that will best meet your requirements and ensure trouble-free performance.

For more information about Portescap stepper motors, visit our product page or explore the full whitepaper here.